One of the processes that is used for the casting of metals is investment casting, commonly known in the art as the lost pattern process. The lost pattern process is often used to create castings of complex shapes, increased dimensional accuracy (such as control of wall thickness), and/or smooth surface characteristics.
In the lost pattern process, a pattern is made and sacrificed when the molten metal is poured. A variety of pattern materials may be used, such as foam, wax, frozen mercury, or frozen water. The material to be used for the pattern depends upon the metal that is to be cast and the specific design considerations for the cast part. The lost pattern process using a foam pattern, i.e., the lost foam process, will be described herein, although it is to be understood that the invention may be used on any known lost pattern process.
The lost foam process involves the injection of foam beads, typically of polystyrene, into a cavity in an aluminum die, where the beads are expanded by steam to fill the cavity. The expanded beads form a pattern that conforms to the shape of the die cavity. Turning to FIG. 1, the pattern 10 is removed from the die cavity and glued to a runner 12 that allows the molten metal to reach the pattern 10 upon pouring. To form a more complex pattern, several individually formed patterns may be glued together.
With reference to FIG. 2, the pattern 10 and runner 12 are dipped into a slurry of ceramic material to form a coating 14 on the pattern 10. The coating 14 is dried and the pattern 10 with the runner 12 and coating 14 is lowered into a flask 16, as shown in FIG. 3. The flask 16 is filled with a backing material such as unbonded sand 18 that is packed around the pattern 10, often by vibration. The vibration allows the sand 18 to penetrate and support the entire pattern 10 and runner 12. A portion of the runner 12 extends to the top 20 of the flask 16 to facilitate the pouring of molten metal.
Turning to FIG. 4, a crucible 22, or similar vessel, contains molten metal (not shown) that is poured through the runner 12 and into the pattern 10. As the molten metal contacts the foam of the runner 12 and the pattern 10, the foam rapidly decomposes and is vaporized. The molten metal thus replaces the foam and the ceramic coating 14 maintains the desired shape and surface characteristics for the casting. The unbonded sand 18 supports the coating 14 to control the dimensional stability of the ceramic coating 14, and thus of the cast part.
The flask 16 is set aside to allow the cast part to cool and solidify, also known as freezing. Once cooling is complete, as FIG. 5 illustrates, the cast part 24, including a gate 26 to be trimmed, is removed from the sand 18 either by extracting the part 24 from the sand 18 or dumping the sand 18 out of the flask 16. The sand 18 is typically reclaimed and re-used. The ceramic coating 14 (referring back to FIG. 4) is removed from the cast part 24 by tumbling or another operation known to those skilled in the art.
However, the lost foam process has some well-known disadvantages. For one, the tooling is highly complex and therefore expensive. Complex parts, such as cylinder heads and blocks, can only be made by specialist tool makers. For these reasons, the process is generally limited to those parts requiring long production runs.
Also, the contact of molten metal with the foam of the pattern causes the evolution of undesirable fumes, creates contamination of the backing aggregate as the styrene decomposes and prematurely cools the molten metal. In addition, as the styrene foam decomposes, it may release hydrogen, which is captured by the molten metal and thus creates significant defects. A lack of uniformity of the density distribution in the foam may also prevent a smooth or predictable filling of the mold, allowing the molten metal to advance more rapidly in some sections of the mold and then fold back as other sections fill, thereby enfolding defects.
These disadvantages have been reduced by a number of alternative techniques in the lost foam casting process. For example, once the pattern is in the flask and supported by the backing material, the foam may be removed before the molten metal is poured. This elimination of the foam before pouring the molten metal may be performed according to the Replicast Process, as known in the art. In this manner, the disadvantages associated with the molten metal contacting the foam may be reduced or eliminated.
Other processes, such as counter-gravity filling (disclosed in U.S. Pat. No. 6,103,182) and the application of pressure after pouring the molten metal, have also reduced some of the disadvantages of the lost foam process.
Still, the foam patterns are relatively weak and must withstand handling and being dipped in the ceramic slurry. This causes designs of patterns to focus on strength rather than better filling, thereby sacrificing optimum casting process characteristics. The weakness of foam patterns also often leads to distortion of the patterns when the backing material is poured around the pattern in the flask. Such weakness of the patterns leads to a need for a coating that may lend more structural support to the patterns.
Other disadvantages of the lost foam casting process are associated with the slow cooling of the cast metal. As mentioned above, after the molten metal is poured into the mold, the mold is typically set aside until enough heat has been lost from the metal so that it has solidified, whereupon the casting is removed from the mold.
The sand that serves as the backing material in lost foam casting is most commonly silica. However, silica experiences an undesirable transition from alpha quartz to beta quartz at about 570 degrees Celsius (° C.), or 1,058 degrees Fahrenheit (° F.). In addition, a silica backing aggregate typically does not allow rapid cooling of the molten metal due to its relatively low thermal conductivity.
Rapid cooling of the molten metal is often desirable, as it is known in the art that with such cooling the mechanical properties of the casting are improved. Moreover, rapid cooling allows the retention of more of the alloying elements in solution, thereby introducing the possibility of eliminating subsequent solution treatment, which saves time and expense. The elimination of solution treatment prevents the quench that typically follows, removing the problems of distortion and residual stress in the casting that are caused by the quench.
As a result, it is desirable to develop a lost foam casting process and related apparatus that provide the advantages of increased structural support of the pattern and more rapid solidification of the cast metal.